1. Field of the Invention
This invention relates to a hydraulic control apparatus for a vehicle, and more particularly to a hydraulic control apparatus which is adapted to be used for steering control of an industrial vehicle such as a shovel loader or the like.
2. Description of the Prior Art
A hydraulic control apparatus of such type which as been conventionally known in the art is typically disclosed in Japanese Patent Application Laying-Open Publication No. 80038/1983 and constructed in such a manner as shown in FIG. 1. More particularly, the conventional hydraulic control apparatus generally comprises a main body 200 having a steering spool 202 and a first flow control valve spool 204 arranged therein and a subsidiary body 206 having a second flow control valve spool 208 arranged therein.
The steering spool 202 is connected at one end thereof to a pilot chamber 210 and at the other end thereof to a pilot chamber 212 a centering spring 214 is arranged to act on the one end of the steering spool 202.
The pilot chambers 210 and 212 are connected through passages 216 and 218 to a totally-hydraulic power steering unit 220. The power steering unit 220 is also connected to a pump 222 and a tank 224 and supplies a portion of oil discharge from the pump 222 proportional to the number of revolutions of a handle 226 to one of the passages 216 and 218, while the other passage communicates with the tank 224.
The first flow control valve spool 204 is connected at one end thereof to a pilot chamber 228 and engaged at the other end thereof with a spring 230 which acts on the spool 204 to normally hold it at a position shown in FIG. 1. Also, the main body 200 is so constructed that when oil discharged from a pump 232 flows into an inlet port 234 formed in the main body 200, discharge pressure of the pump 232 is transmitted through an oil passage 236 formed in the first flow control valve spool 204 to the spool 204. Thus, the first flow control valve spool 204 is moved against the spring 230, so that the inlet port 234 may communicate with a communication passage 238 in the main body 200.
The second flow control valve spool 208 inorporated in the subsidiary body 206 is positioned at one end thereof in a pilot chamber 240 and engaged at the other end thereof with a spring 242 so that it may be normally held at such a position as shown in FIG. 1. The pilot chamber 240 is selectively connected through a shuttle valve 244 to one of the passages 216 and 218 having higher pressure so that it may be transmitted or applied to the pilot chamber 240.
When the pump 232 is driven, discharge pressure of the pump 232 is transmitted to the pilot chamber 228, resulting in the first flow control valve spool 204 being moved against the spring 230. This causes oil discharged from the pump 232 to flow into the communication passage 238. However, at this time, when the handle 226 is at a neutral position, the steering spool 202 is kept at the normal position shown in FIG. 1 to keep the communication passage 238 closed.
When the handle 226 is at the neutral position described above, pressure is not applied to the pilot chamber 240 of the subsidiary body 206 as well, so that the second flow control valve spool 208 may be kept at a normal position shown in FIG. 1. Accordingly, when oil discharged from a pump 248 flows into a port 246 formed in the subsidiary body 206, all the discharged oil flows into a drive circuit 250 on an implement side and merges with oil discharged from a pump 252 for supplying pressurized oil to the drive circuit 250.
In such a situation as described above, when the handle 226 is rotated, for example, in a clockwise direction, oil is discharged in an amount proportional to the number of revolutions of the handle 226 to the passage 216. The passages 216 and 218 are formed so as to communicate with each other through an orifice 254, so that the oil discharged to the passage 216 may be passed through the orifice 254. Passing of the oil through the orifice 254 causes a pressure difference to occur at the orifice 254. The pressure difference is varied depending on the amount of oil discharged from the power steering unit 220 or the number of revolutions of the handle 226.
The above-described pressure difference at the orifice 254 forms a pressure difference between the pilot chambers 210 and 212 in which both ends of the steering spool 202 are arranged, so that the amount of change-over of the steering spool 202 may be controlled depending on the number of revolutions of the handle 226. More specifically, rotation of the handle 226 in a large amount in the clockwise direction in FIG. 1 increases the pressure difference to cause the steering spool 202 to be moved or shifted by a large distance in a right direction in FIG. 1, whereas the rotation in a small amount causes the amount of shift of the spool to be decreased. When the amount of shift or movement of the steering spool 202 is increased, the degree of opening of the variable constriction formed by cooperation between the communication passage 238 and notches 256 formed at the spool 202 is increased, whereas a decrease in the amount of shift of the spool 202 causes the degree of opening of the constriction to be decreased.
Pressure on a downstream side of the variable constriction is applied through a through-hole 258 and a port 260 to a spring chamber 262.
For example, a decrease in the number of revolutions of the handle 226 causes the degree of opening of the variable constriction to be decreased, resulting in a pressure difference at the variable constriction being increased. This leads to an increase in a difference between pressure in the pilot chamber 228 to which pressure on the upstream side of the variable constriction is applied and pressure in the spring chamber 262 to which pressure on a downstream side of the variable constriction is applied. This causes the first flow control valve spool 204 to be substantially shifted against the spring 230 to communicate the port 234 with a port 264 formed in the main body 200.
Further, discharge of pressure oil from the pump 222 to the passage 216 as described above causes discharge pressure of the pump to be transmitted through the shuttle valve 244 to the pilot chamber 240, so that the second flow control valve spool 208 may be moved or shifted against the spring 242, resulting in the port 246 in the subsidiary body 206 communication with the port 234 in the main body 200.
Thus, the above-described operation causes oil discharged from the pump 232 to merge with that discharged from the pump 248. However, the degree of opening of the variable constriction is small, accordingly, the amount of oil supplied to a steering cylinder 266 is substantially limited and most of the oil is supplied through the port 264 to the drive circuit 250 on the implement side.
On the contrary, when the number of revolutions of the handle 226 is increased, the amount of oil discharged from the steering unit 220 is increased correspondingly, resulting in a pressure difference at the orifice 254 being increased. This leads to an increase in the amount of movement of the steering spool 202, so that the degree of opening of the variable constriction may be increased. Such an increase in the degree of opening of the variable constriction decreases a pressure difference at the variable constriction to reduce the amount of movement the first flow control valve spool 204, to thereby interrupt communication between the port 234 and the port 264.
Even in this instance, pilot pressure is applied to the pilot chamber 240, so that the second flow control valve spool 208 may be shifted against the spring 242 to accomplish communication betweet the port 234 and the port 264.
Thus, all oil discharged from the pump 232 and 248 is supplied to the steering cylinder 266.
As can be seen from the foregoing, in the conventional hydraulic control apparatus, a decrease in the number of revolutions of the handle 226 causes a part of oil discharged from the pump 232 to be fed to the steering cylinder 266, and the remaining oil and all oil discharged from the pump 248 are supplied to the drive circuit 250 on the implement side.
The more the number of revolutions of the handle 226 is increased, the more the amount of oil supplied to the steering cylinder 266 is increased, and when the amount of oil supplied to the cylinder 266 reaches a predetermined level, surplus oil from the pumps 232 and 248 is fed to the implement side.
Unfortunately, as can be seen from the foregoing, the conventional hydraulic control apparatus constructed as described above requires two such flow control valves, resulting in a complicated structure. Also, when there are variations in operational or functional characteristics of the flow control valve, the variations synergistically affect each other. This causes a problem of rendering control characteristics of the whole apparatus highly unstable.
Accordingly, it would be highly desirable to develop a hydraulic control apparatus for a vehicle which is capable of being significantly simplified in structure and stabilizing its control characteristics.